Symmetric sweep phase sweep transmit diversity

ABSTRACT

Described herein is a method and apparatus for transmission that provides the performance of space time spreading (STS) or orthogonal transmit diversity (OTD) and the backwards compatibility of phase sweep transmit diversity (PSTD) without degrading performance of either STS or PSTD using a symmetric sweep PSTD transmission architecture. In one embodiment, a pair of signals s 1  and s 2  are split into signals s 1 (a) and s 1 (b) and signals s 2 (a) and s 2 (b), respectively. Signal s 1  comprises a first STS/OTD signal belonging to an STS/OTD pair, and signal s 2  comprises a second STS/OTD signal belonging to the STS/OTD pair. Signals s 1 (b) and s 2 (b) are phase swept using a pair of phase sweep frequency signals that would cancel out any self induced interference. For example, the pair of phase sweep frequency signals utilize a same phase sweep frequency with one of the phase sweep frequency signals rotating in the opposite direction plus an offset of π relative to the other phase sweep frequency signal. The resultant phase swept signals s 1 (b) and s 2 (b) are added to signals s 2 (a) and s 1 (a) before being amplified and transmitted.

RELATED APPLICATION

Related subject matter is disclosed in the following applications filedconcurrently and assigned to the same assignee hereof: U.S. patentapplication Ser. No. 09/918,391 entitled, “Space Time Spreading andPhase Sweep Transmit Diversity”, inventors Roger Benning, R. MichaelBuehrer, Paul A Polakos and Rober Atmaram Soni; U.S. application Ser.No. 09/918,393 entitled, “Biased Phase Sweep Transmit Diversity”,inventors Roger Benning, R. Michael Buehrer and Robert Atmaram Soni; andU.S. patent application Ser. No. 09/918,086 entitled, “Split Shift PhaseSweep Transmit Diversity”, inventors Roger Benning, R. Michael Buehrer,Robert Atmaram Soni and Paul A. Polakos.

BACKGROUND OF THE RELATED ART

Performance of wireless communication systems is directly related tosignal strength statistics of received signals. Third generationwireless communication systems utilize transmit diversity techniques fordownlink transmissions (i.e., communication link from a base station toa mobile-station) in order to improve received signal strengthstatistics and, thus, performance. Two such transmit diversitytechniques are space time spreading (STS) and phase sweep transmitdiversity (PSTD).

FIG. 1 depicts a wireless communication system 10 employing STS.Wireless communication system 10 comprises at least one base station 12having two antenna elements 14-1 and 14-2, wherein antenna elements 14-1and 14-2 are spaced far apart for achieving transmit diversity. Basestation 12 receives a signal S for transmitting to mobile-station 16.Signal S is alternately divided into signals S_(e) and s_(o) whereinsignal s_(e) comprises even data bits and signal s_(o) comprises odddata bits. Signals s_(e) and s_(o) are processed to produce signalsS¹⁴⁻¹ and S¹⁴⁻². Specifically, s_(e) is multiplied with Walsh code w₁ toproduce signal s_(e)w₁; a conjugate of signal s_(o) is multiplied withWalsh code w₂ to produce signal s_(o)*w₂; signal s_(o) is multipliedwith Walsh code w₁ to produce s_(o)w₁; and a conjugate of signal s_(e)is multiplied with Walsh code w₂ to produce s_(e)*w₂. Signal s_(e)w₁ isadded to signal s_(o)*w₂ to produce signal S¹⁴⁻¹ (i.e.,S¹⁴⁻¹=s_(e)w₁+s_(o)*w₂) and signal s_(e)*w₂ is subtracted from signals_(o)w₁ to produce signal S¹⁴⁻² (i.e., S¹⁴⁻²=s_(o)w₁−s_(e)*w₂). SignalsS¹⁴⁻¹ and S¹⁴⁻² are transmitted at substantially equal or identicalpower levels over antenna elements 14-1 and 14-2, respectively. Forpurposes of this application, power levels are “substantially equal” or“identical” when the power levels are within 1% of each other.

Mobile-station 16 receives signal R comprising γ₁(S¹⁴⁻²)+γ₂(S¹⁴⁻²),wherein γ₁ and γ₂ are distortion factor coefficients associated with thetransmission of signals S¹⁴⁻¹ and S¹⁴⁻² from antenna elements 14-1 and14-2 to mobile-station 16, respectively. Distortion factor coefficientsγ₁ and γ₂ can be estimated using pilot signals, as is well-known in theart. Mobile-station 16 decodes signal R with Walsh codes w₁ and w₂ torespectively produce outputs:W ₁=γ₁ s _(e)+γ₂ s _(o)tm equation 1W ₂=γ₁ s _(o)*−γ₂ s _(e)*  equation 1aUsing the following equations, estimates of signals s_(e) and s_(o),i.e., ŝ_(e) and ŝ_(o), may be obtained:ŝ _(e)=γ₁ *W ₁−γ₂ W ₂ *=s _(e)(|γ₁|²+|γ₂|²)+noise  equation 2ŝ _(o)=γ₂ *W ₁+γ₁ W ₂ *=s _(o)(|γ₁|²+|γ₂|²)+noise′   equation 2a

However, STS is a transmit diversity technique that is not backwardcompatible from the perspective of the mobile-station. That is,mobile-station 16 is required to have the necessary hardware and/orsoftware to decode signal R. Mobile-stations without such hardwareand/or software, such as pre-third generation mobile-stations, would beincapable of decoding signal R.

By contrast, phase sweep transmit diversity (PSTD) is backwardcompatible from the perspective of the mobile-station. FIG. 2 depicts awireless communication system 20 employing PSTD. Wireless communicationsystem 20 comprises at least one base station 22 having two antennaelements 24-1 and 24-2, wherein antenna elements 24-1 and 24-2 arespaced far apart for achieving transmit diversity. Base station 22receives a signal S for transmitting to mobile-station 26. Signal S isevenly power split into signals s₁ and s₂ and processed to producesignals S²⁴⁻¹ and S²⁴⁻², where s₁=s₂. Specifically, signal s₁ ismultiplied by Walsh code w_(k) to produce S²⁴⁻¹=s₁w_(k), where krepresents a particular user or mobile-station. Signal s₂ is multipliedby Walsh code w_(k) and a phase sweep frequency signal e^(j2πf) ^(s)^(t) to produce S²⁴⁻², i.e., S₂₄₋₂=s₂w_(k)e^(j2πf) ^(s)^(t)=s₁w_(k)e^(j2πf) ^(s) ^(t)=S²⁴⁻¹e^(j2πf) ^(s) ^(t), where f_(s) is aphase sweep frequency and t is time.

Signals S²⁴⁻¹ and S₂₄₋₂ are transmitted at substantially equal powerlevels over antenna elements 24-1 and 24-2, respectively. Note that thephase sweep signal e^(j2πf) ^(r) ^(t) is being represented in complexbaseband notation, i.e., e^(j2πf) ^(s)^(t)=cos(2πf_(s)t)+jsin(2πf_(s)t). It should be understood that thephase sweep signal may also be applied at an intermediate frequency or aradio frequency.

Mobile-station 26 receives signal R comprising γ₁S²⁴⁻¹+γ₂S²⁴⁻².Simplifying the equation for R results inR=γ ₁ S ²⁴⁻¹+γ₂ S ²⁴⁻¹ e ^(j2πf) ^(s) ^(t)tm equation 3R=S ²⁴⁻¹{γ₁+γ₂ e ^(j2πf) ^(s) ^(t)}  equation 3aR=S ²⁴⁻¹γ_(eq)tm equation 3bwhere γ_(eq) is an equivalent channel seen by mobile-station 26.Distortion factor coefficient γ_(eq) can be estimated using pilotsignals and used, along with equation 3b, to obtain estimates of signals₁ and/or S₂.

In slow fading channel conditions, both transmit diversity techniques,i.e., STS and PSTD, improve performance (relative to when no transmitdiversity technique is used) by making the received signal strengthstatistics associated wit a slow fading channel at the receiver looklike those associated with a fast fading channel. However, PSTD does notprovide the same amount of overall performance improvement as STS.Accordingly, there exists a need for a transmission technique thatprovides the performance of STS and the backwards compatibility of PSTDwithout degrading performance of either STS or PSTD.

SUMMARY OF THE INVENTION

The present invention is a method and apparatus for transmission thatprovides the performance of space time spreading (STS) or orthogonaltransmit diversity (OTD) and the backwards compatibility of phase sweeptransmit diversity (PSTD) without degrading performance of either STS orPSTD using a symmetric sweep PSTD transmission architecture, whichinvolves phase sweeping a pair of signals having a pair of STS/OTDsignals. In one embodiment, a pair of signals s₁ and s₂ are split intosignals s₁(a) and s₁(b) and signals s₂(a) and s₂(b), respectively.Signal s₁ comprises a first STS/OTD signal belonging to an STS/OTD pair,and signal s₂ comprises a second STS/OTD signal belonging to the STS/OTDpair. Signals s₁(b) and s₂(b) are phase swept using a pair of phasesweep frequency signals that would cancel out any self inducedinterference caused by phase sweeping both signals s₁(b) and s₂(b). Forexample, the pair of phase sweep frequency signals utilize a same phasesweep frequency with one of the phase sweep frequency signals rotatingin the opposite direction plus an offset of π relative to the otherphase sweep frequency signal. The resultant phase swept signals s₁(b)and s₂(b) are added to signals s₂(a) and s₁(a) before being amplifiedand transmitted.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, aspects, and advantages of the present invention willbecome better understood with regard to the following description,appended claims, and accompanying drawings where

FIG. 1 depicts a wireless communication system employing space timespreading techniques in accordance with the prior art;

FIG. 2 depicts a wireless communication system employing phase sweeptransmit diversity in accordance with the prior art; and

FIG. 3 depicts a base station employing symmetric sweep phase sweeptransmit diversity in accordance with one embodiment of the presentinvention;

FIG. 4 depicts a base station employing symmetric sweep phase sweeptransmit diversity in accordance with another embodiment of the presentinvention; and

FIG. 5 depicts a base station employing symmetric sweep phase sweeptransmit diversity in accordance with another embodiment of the presentinvention.

DETAILED DESCRIPTION

FIG. 3 depicts a base station 30 employing symmetric sweep phase sweeptransmit diversity in accordance with the present invention, whereinsymmetric sweep phase sweep transmit diversity utilizes code divisionmultiple access (CDMA), phase sweep transmit diversity (PSTD), and spacetime spreading (STS) or orthogonal transmit diversity (OTD) techniques.CDMA, PSTD, STS and OTD are well-known in the art.

Base station 30 provides wireless communication services tomobile-stations, not shown, in its associated geographical coverage areaor cell, wherein the cell is divided into three sectors α, β, γ. Notethat the base station could be divided into an arbitrary number ofsectors and not change the invention described here. Base station 30includes a transmission architecture that incorporates STS or OTD andbiased PSTD, as will be described herein.

Base station 30 comprises a processor 32, splitters 34, 35, multipliers36, 38, 40, 41, adders 42, 43, amplifiers 44, 46, and a pair ofdiversity antennas 48, 50. Note that base station 30 also includesconfigurations of splitters, multipliers, adders, amplifiers andantennas for sectors β, γ that are identical to those for sector α. Forsimplicity sake, the configuration for sectors β, γ are not shown.Additionally, for discussion purposes, it is assumed that signals S_(k)are intended for mobile-stations k located in sector α and, thus, thepresent invention will be described with reference to signals S_(k)being processed for transmission over sector α.

Processor 32 includes software for processing signals S_(k) inaccordance with well-known CDMA and STS/OTD techniques. The manner inwhich a particular signal S_(k) is processed by processor 32 depends onwhether mobile-station k is STS/OTD compatible, i.e., mobile-stationcapable of decoding signals processed using STS/OTD. Processor 32 mayalso include software for determining whether mobile-station k isSTS/OTD compatible. If mobile-station k is not STS/OTD compatible, thensignal S_(k) is processed in accordance with CDMA techniques to producesignal S_(k-1), which is also referred to herein as a non-STS/OTD signalS_(k-1).

Note that, in another embodiment, processor 32 is operable to processsignals S_(k) in accordance with a multiple access technique other thanCDMA, such as time or frequency division multiple access. In thisembodiment, when mobile-station k is not STS/OTD compatible, then signalS_(k) is processed in accordance with such other multiple accesstechnique to produce the non-STS/OTD signal S_(k-1).

If mobile-station k is STS/OTD compatible, then signal S_(k) isprocessed in accordance with CDMA and STS/OTD. Specifically, ifmobile-station k is STS compatible, then signal S_(k) is processed usingSTS. Such process includes alternately dividing signal S_(k) intosignals s_(e) and s_(o), wherein signal s_(e) comprises even data bitsand signal s_(o) comprises odd data bits. Signal s_(e) is multipliedwith Walsh code w₁ to produce signal s_(e)w₁, and a conjugate of signals_(e) is multiplied with Walsh code w₂ to produce s_(e)*w₂. Signal s_(o)is multiplied with Walsh code w₁ to produce s_(o)w₁, and a conjugate ofsignal s_(o) is multiplied with Walsh code w₂ to produce signals_(o)*w₂. Signal s_(e)w₁ is added to signal s_(o)*w₂ to produce signalS_(k-2)(a)=s_(e)w₁+s_(o)*w₂. Signal s_(e)*w₂ is subtracted from signals_(o)w₁ to produce signal S_(k-2)(b)=s_(o)w₁−s_(e)*w₂. SignalsS_(k-2)(a), S_(k-2)(b) are also referred to herein as STS signals, andtogether signals S_(k-2)(a), S_(k-2)(b) collectively comprise an STSpair.

If mobile-station k is OTD compatible, then signal S_(k) is processedusing OTD. Orthogonal transmit diversity involves dividing signal S_(k)into signals s_(e) and s_(o), and multiplying signals s_(e) and s_(o)using Walsh codes w₁, w₂ to produce signals S_(k-3)(a), S_(k-3)(b),i.e., S_(k-3)(a)=s_(e)w₁ and S_(k-3)(b)=s_(o)w₂, respectively. SignalsS_(k-3)(a), S_(k-3)(b) are also referred to herein as OTD signals, andtogether signals S_(k-3)(a), S_(k-3)(b) collectively comprise an OTDpair.

For illustration purposes, the present invention will be describedherein with reference to STS and signals S_(k-2)(a), S_(k-2)(b). Itshould be understood that the present invention is also applicable toOTD and signals S_(k-3)(a), S_(k-3)(b).

The output of processor 32 are signals s_(α-1), s_(α-2), where signals_(α-1) comprises of signals S_(k-1) and S_(k-2)(a) and signal s_(α-2)comprises of signals S_(k-2)( b), i.e., s_(α-1)=ΣS_(k-1)+ΣS_(k-2)(a) ands_(α-2)=ΣS_(k-2)(b). That is, signals intended for STS compatiblemobile-stations are included in both output signals s_(α-1), s_(α-2) andsignals intended for non-STS compatible mobile-stations are included inonly signal s_(α-1). Alternately, signal s_(α-1) comprises of signalsS_(k-1) and S_(k-2)( b) and signal s_(α-2) comprises of signalsS_(k-2)(a).

Signal s_(α-1) is split by splitter 34 into signals s_(α-1)(a),s_(α-1)(b) and processed along paths A1 and B1, respectively, bymultipliers 36, 38, 40, adders 42, 43 and amplifiers 44, 46 inaccordance with PSTD techniques. Signal s_(α-2) is split by splitter 35into signals s_(α-2)(a), s_(α-2)(b) and processed along paths A2 and B2,respectively, by multipliers 38, 40, 41, adders 42, 43 and amplifiers44, 46 in accordance with PSTD techniques. Note that signals s_(α-1)(a),s_(α-2)(a) are identical to respective signal s_(α-1)(b), s_(α-2)(b) interms of data, and that signals s_(α-1), s_(α-2) may be evenly orunevenly split in terms of power.

Signals s_(α-1)(b), s_(α-2)(b) are provided as inputs into multipliers36, 41 where signals s_(α-1)(b), s_(α-2)(b) are frequency phase sweptwith phase sweep frequency signals (JIMMY: I can't edit the equations,but change all of the “−” signs in the exponents to “+” signs in ALLe^(j) terms. Please change this in all of the figures as well. e^(jΘ)^(s) ^((t)), e^(jΘ) ^(s2) ^((t)) to produce signals S₃₆=s_(α-1)(b)e^(jΘ)^(s) ^((t)), S₄₁=s_(α-2)(b)e^(jΘ) ^(s2) ^((t)), respectively, whereinΘ_(s)=2πf_(s)t, e^(jΘ) ^(s) ^((t))=cos(2πf_(s)t)+jsin(2πf_(s)t),Θ_(s2)=−2πf_(s)t+π, e^(jΘ) ^(s2) ^((t))=−cos(2πf_(s)t)+jsin(2πf_(s)t),f_(s) represents a fixed or varying phase sweep frequency and trepresents time.

Note that phase sweep frequency signals e^(jΘ) ^(s) ^((t)), e^(jΘ) ^(s2)^((t)) utilize a same phase sweep frequency with one of the signals,i.e., e^(jΘ) ^(s2) ^((t)), rotating in the opposite direction plus anoffset of π relative to the other signal, i.e., e^(jΘ) ^(s) ^((t)). Ifthe phase sweep frequency signals e^(jΘ) ^(s) ^((t)), e^(jΘ) ^(s2)^((t)) were identical, i.e., Θ_(s)=Θ_(s2), self induced interferencewould be generated by base station 30 that would degrade STS/OTDperformance. By configuring the phase sweep signals e^(jΘ) ^(s) ^((t)),e^(jΘ) ^(s2) ^((t)) to have this relationship, the self inducedinterference is canceled and STS/OTD performance is optimized.

Signal S₄₁ is added to signal s_(α-1)(a) by adder 43 to produce signalS₄₃=S₄₁+s_(α-1)(a)=s_(α-2)(b)e^(jΘ) ^(s2) ^((t))+s_(α-1)(a). Signal S₄₃and carrier signal e^(j2πf) ^(c) ^(t) are provided as inputs intomultiplier 40 to produce signal S₄₀, where S₄₀=(S_(α-2)(b)e^(jΘ) ^(s2)^((t))+s_(α-1)(a))e^(j2πf) ^(c) ^(t), e^(j2πf) ^(c) ^(t)=cos(2πf_(c)^(t))+jsin(2πf_(c)t, and f_(c) represents a carrier frequency.

Signal S₃₆ is added to signal s_(α-2)(a) by adder 42 to produce signalS₄₂=s_(α-1)(b)e^(jΘ) ^(s) ^((t))+s_(α-2)(a). Signal S₄₂ and carriersignal e^(j2πf) ^(c) ^(t) are provided as inputs into multiplier 38 toproduce signal S₃₈, where S₃₈=(s_(α-1)(b)e^(jΘ) ^(s)^((t))+s_(α-2)(a))e^(j2πf) ^(c) ^(t).

Signals S₄₀, S₃₈ are amplified by amplifiers 44, 46 to produce signalsS₄₄ and S₄₆ for transmission over antennas 48, 50, where signalS₄₄=A₄₄((s_(α-2)(b)e^(jΘ) ^(s2) ^((t))+s_(α-1)(a))e^(j2πf) ^(t) ),S₄₆=A₄₆(s_(α-1)(b)e^(jΘ) ^(s) ^((t))+s_(α-2)(a))e^(j2πf) ^(c) ^(t), A₄₄represents the amount of gain associated with amplifier 44 and A₄₆represents the amount of gain associated with amplifier 46.

In one embodiment, the amounts of gain A₄₄, A₄₆ are substantially equal.In this embodiment, signals s_(α-1), s_(α-2) are split by splitters 34,35 such that the power levels of signals s_(α-1)(a), s_(α-2)(a) aresubstantially equal to the power levels of signal s_(α-1)(b),s_(α-2)(b). Advantageously, equal gain amplifiers can be used, whichlowers the cost of base station 30 compared to base station cost whenunequal amplifiers are used.

In another embodiment, the amounts of gain A₄₄, A₄₆ are different andrelated to how splitters 34, 35 split signals s_(α-1), s_(α-2).Specifically, the amounts of gain A₄₄, A₄₆ applied to signals S₄₀, S₃₈should be amounts that would cause the power levels of signals S₄₄ andS₄₆ to be approximately or substantially equal. For purposes of thisapplication, power levels are “approximately equal” when the powerlevels are within 10% of each other.

FIG. 5 depicts a base station 70 employing symmetric sweep phase sweeptransmit diversity in accordance with one embodiment of the presentinvention. In this embodiment, a form of PSTD referred to herein assplit shift PSTD in also utilized. Spilt shift PSTD involves shiftingboth signals split from a single signal using phase sweep frequencysignals that sweeps both signals in opposite direction. As shown in FIG.5, signals s_(α-1)(a), s_(α-2)(a) are phase swept by multipliers 37, 39using phase sweep frequency signals e^(−jΘ) ^(s) ^((t)), e^(−jΘ) ^(s2)^((t)), respectively. Although this embodiment depicts phase sweepfrequency signals e^(−jΘ) ^(s) ^((t)), e^(−jΘ) ^(s2) ^((t)) equal andopposite to phase sweep frequency signals e^(jΘ) ^(s) ^((t)), e^(jΘ)^(s2) ^((t)), it should be understood that the phase sweep frequencysignals used to phase sweep signals s_(α-1)(a), s_(α-2)(a) need not beequal in magnitude. In another embodiment, signals s_(α-1)(a),s_(α-2)(a) are phase swept using phase sweep frequency signals thatresult in phase swept signals s_(α-1)(a), s_(α-2)(a) with a desired orother phase difference to phase swept signals s_(α-1)(b), s_(α-2)(b).Note that that the phase sweep frequency signal used to phase sweepsignals s_(α-1)(a), s_(α-2)(a), s_(α-1)(b), s_(α-2)(b) may be phaseshifting at an identical or different rate from each other, may be phaseshifting at fixed and/or varying rates, or may be phase shifting in thesame or opposite direction.

Although the present invention has been described in considerable detailwith reference to certain embodiments, other versions are possible. Forexample, phase sweeping could be performed on paths A1 and/or A2 insteadof paths B1 and/or B2. In another example, the phase sweep frequencysignals are interchanged. FIG. 4 depicts another embodiment of thepresent invention in which phase sweeping is performed along paths A1and A2 instead of paths B1 and B2 and phase sweep frequency signalse^(jΘ) ^(s) ^((t)), e^(jΘ) ^(s2) ^((t)) are provided as inputs intomultipliers 41, 36, respectively. Therefore, the spirit and scope of thepresent invention should not be limited to the description of theembodiments contained herein.

1. A method of signal transmission comprising the steps of: splitting asignal s₁ into signals s₁(a) and s₁(b), wherein signal s₁ comprises afirst STS/OTD signal belonging to an STS/OTD pair; splitting a signal s₂into signals s₂(a) and s₂(b), wherein signal s₂ comprises a secondSTS/OTD signal belonging to the STS/OTD pair; phase sweeping the signals₁(b) using a first phase sweep frequency signal to produce a phaseswept signal s₁(b); phase sweeping the signal s₂(b) using a second phasesweep frequency signal to produce a phase swept signal s₂(b), the firstand second phase sweep frequency signals being configured to cancel outany self induced interference caused by phase sweeping the signals s₁(b)and s₂(b); adding the phase swept signal s₂(b) to the signal s₁(a) toproduce a summed signal s₃; and adding the phase swept signal s₁(b) tothe signal s₂(a) to produce a summed signal s₄.
 2. The method of claim1, wherein the first and second phase sweep frequency signals utilize asame phase sweep frequency with the second phase sweep frequency signalrotating in the opposite direction plus an offset of π relative to thefirst phase sweep frequency signal.
 3. The method of claim 1, whereinthe first and second phase sweep frequency signals utilize a same phasesweep frequency with the first phase sweep frequency signal rotating inthe opposite direction plus an offset of π relative to the second phasesweep frequency signal.
 4. The method of claim 1 comprising theadditional steps of: amplifying the summed signal s₃ to produce anamplified summed signal s₃; and amplifying the summed signal s₄ toproduce an amplified summed signal s₄.
 5. The method of claim 1comprising the additional steps of: transmitting the summed signal s₃over a first antenna belonging to a pair of diversity antennas; andtransmitting the summed signal s₄ over a second antenna belonging to thepair of diversity antennas.
 6. The method of claim 1 comprising theadditional steps of: prior to the step of adding the phase swept signals₂(b) to the signal s₁(a), phase sweeping the signal s₁(a) using a thirdphase sweep frequency signal to produce a phase swept signal s₁(a) witha different phase from the phase swept signal s₂(b); and prior to thestep of adding the phase swept signal s₁(b) to the signal s₂(a), phasesweeping the signal s₂(a) using a fourth phase sweep frequency signal toproduce a phase swept signal s₂(a) with a different phase from the phaseswept signal s₁(b).
 7. A method of signal transmission comprising thesteps of: splitting a signal s₁ into signals s₁(a) and s₁(b), whereinsignal s₁ comprises a first STS/OTD signal belonging to an STS/OTD pair;splitting a signal s₂ into signals s₂(a) and s₂(b), wherein signal s₂comprises a second STS/OTD signal belonging to the STS/OTD pair; phasesweeping the signal s₁(a) using a first phase sweep frequency signal toproduce a phase swept signal s₁(a); phase sweeping the signal s₂(a)using a second phase sweep frequency signal to produce a phase sweptsignal s₂(a), the first and second phase sweep frequency signals beingconfigured to cancel out any self induced interference caused by phasesweeping the signals s₁(a) and s₂(a); adding the phase swept signals₂(a) to the signal s₁(b) to produce a summed signal s₃; and adding thephase swept signal s₁(a) to the signal s₂(b) to produce a summed signals₄.
 8. The method of claim 7, wherein the first and second phase sweepfrequency signals utilize a same phase sweep frequency with the secondphase sweep frequency signal rotating in the opposite direction plus anoffset of π relative to the first phase sweep frequency signal.
 9. Themethod of claim 7, wherein the first and second phase sweep frequencysignals utilize a same phase sweep frequency with the first phase sweepfrequency signal rotating in the opposite direction plus an offset of πrelative to the second phase sweep frequency signal.
 10. The method ofclaim 7 comprising the additional steps of: amplifying the summed signals₃ to produce an amplified summed signal s₃; and amplifying the summedsignal s₄ to produce an amplified summed signal s₄.
 11. The method ofclaim 7 comprising the additional steps of: transmitting the summedsignal s₃ over a first antenna belonging to a pair of diversityantennas; and transmitting the summed signal s₄ over a second antennabelonging to the pair of diversity antennas.
 12. The method of claim 7comprising the additional steps of: prior to the step of adding thephase swept signal s₂(a) to the signal s₁(b), phase sweeping the signals₁(b) using a third phase sweep frequency signal to produce a phaseswept signal s₁(b) with a different phase from the phase swept signals₂(a); and prior to the step of adding the phase swept signal s₁(a) tothe signal s₂(b), phase sweeping the signal s₂(b) using a fourth phasesweep frequency signal to produce a phase swept signal s₂(b) with adifferent phase from the phase swept signal s₁(a).
 13. A base stationcomprising: a first splitter for splitting a signal s₁ into signalss₁(a) and s₁(b), wherein signal s₁ comprises a first STS/OTD signalbelonging to an STS/OTD pair; a second splitter for splitting a signals₂ into signals s₂(a) and s₂(b), wherein signal s₂ comprises a secondSTS/OTD signal belonging to the STS/OTD pair, a first multiplier forphase sweeping the signal s₁(b) using a first phase sweep frequencysignal to produce a phase swept signal s₁(b); a second multiplier forphase sweeping the signal s₂(b) using a second phase sweep frequencysignal to produce a phase swept signal s₂(b), the first and second phasesweep frequency signals being configured to cancel out any self inducedinterference caused by phase sweeping the signals s₁(b) and s₂(b); afirst adder for adding the phase swept signal s₂(b) to the signal s₁(a)to produce a summed signal s₃; and a second adder for adding the phaseswept signal s₁(b) to the signal s₂(a) to produce a summed signal s₄.14. The base station of claim 13, wherein the first and second phasesweep frequency signals utilize a same phase sweep frequency with thesecond phase sweep frequency signal rotating in the opposite directionplus an offset of π relative to the first phase sweep frequency signal.15. The base station of claim 13, wherein the first and second phasesweep frequency signals utilize a same phase sweep frequency with thefirst phase sweep frequency signal rotating in the opposite directionplus an offset of π relative to the second phase sweep frequency signal.16. The base station of claim 13 further comprising: a first amplifierfor amplifying the summed signal s₃ to produce an amplified summedsignal s₃; and a second amplifier for amplifying the summed signal s₄ toproduce an amplified summed signal s₄.
 17. The base station of claim 13further comprising: a pair of diversity antennas having a first and asecond antenna; a first transmitter for transmitting the summed signals₃ over the first antenna; and a second transmitter for transmitting thesummed signal s₄ over the second antenna.
 18. The base station of claim13 further comprising: a third multiplier for phase sweeping the signals₁(a) using a third phase sweep frequency signal to produce a phaseswept signal s₁(a) with a different phase from the phase swept signals₂(b) prior to adding the phase swept signal s₂(b) to the signal s₁(a);and a fourth multiplier for phase sweeping the signal s₂(a) using afourth phase sweep frequency signal to produce a phase swept signals₂(a) with a different phase from the phase swept signal s₁(b) prior toadding the phase swept signal s₁(b) to the signal s₂(a).
 19. A basestation comprising: a first splitter for splitting a signal s₁ intosignals s₁(a) and s₁(b), wherein signal s₁ comprises a first STS/OTDsignal belonging to an STS/OTD pair; a second splitter for splitting asignal s₂ into signals s₂(a) and s₂(b), wherein signal s₂ comprises asecond STS/OTD signal belonging to the STS/OTD pair; a first multiplierfor phase sweeping the signal s₁(a) using a first phase sweep frequencysignal to produce a phase swept signal s₁(a); a second multiplier forphase sweeping the signal s₂(a) using a second phase sweep frequencysignal to produce a phase swept signal s₂(a), the first and second phasesweep frequency signals being configured to cancel out any self inducedinterference caused by phase sweeping the signals s₁(a) and s₂(a); afirst adder for adding the phase swept signal s₂(a) to the signal s₁(b)to produce a summed signal s₃; and a second adder for adding the phaseswept signal s₁(a) to the signal s₂(b) to produce a summed signal s₄.20. The base station of claim 19, wherein the first and second phasesweep frequency signals utilize a same phase sweep frequency with thesecond phase sweep frequency signal rotating in the opposite directionplus an offset of π relative to the first phase sweep frequency signal.21. The base station of claim 19, wherein the first and second phasesweep frequency signals utilize a same phase sweep frequency with thefirst phase sweep frequency signal rotating in the opposite directionplus an offset of π relative to the second phase sweep frequency signal.22. The base station of claim 19 further comprising: a first amplifierfor amplifying the summed signal s₃ to produce an amplified summedsignal s₃; and a second amplifier for amplifying the summed signal s₄ toproduce an amplified summed signal s₄.
 23. The base station of claim 19further comprising: a pair of diversity antennas having a first and asecond antenna; a first transmitter for transmitting the summed signals₃ over the first antenna; and a second transmitter for transmitting thesummed signal s₄ over the second antenna.
 24. The base station of claim19 further comprising: a third multiplier for phase sweeping the signals₁(b) using a third phase sweep frequency signal to produce a phaseswept signal s₁(b) with a different phase from the phase swept signals₂(a) prior to adding the phase swept signal s₂(a) to the signal s₁(b);and a fourth multiplier for phase sweeping the signal s₂(b) using afourth phase sweep frequency signal to produce a phase swept signals₂(b) with a different phase from the phase swept signal s₁(a) prior toadding the phase swept signal s₁(a) to the signal s₂(b).